This invention relates to a CODEC system for use in efficiently encoding and decoding an input image signal, such as a television signal, a video signal, which is representative of a sequence of picture elements. It is to be noted throughout the instant specification that the input image signal is given as a sequence of digital image signals and that each of the picture elements is provided in the form of a digital picture element signal produced within a sample period.
As well known in the art, an image is displayed on a display device in response to an input image signal and is structured by a preselected number of scanning lines divisible into odd number scanning lines and even number scanning lines. In this connection, it is to be noted here that the input image signal is also divisible into an odd number line signal and an even number line signal for the odd number line and the even number line, respectively. When the input image signal is a television signal used in an NTSC system, the odd number line signal and the even number line signal appear within each of fields.
In order to efficiently encode and decode such an input image signal, a CODEC system of the type described often comprises a predictive encoding device and a predictive decoding device which carry out predictive encoding and predictive decoding of the input image signal. In each of the predictive encoding device and the predictive decoding device, a local decoded signal or a local predictive signal should be produced therein before prediction is carried out as regards a current picture element of a current scanning line. Such a local decoded signal or a local predictive signal is derived from a previous picture element received with a previous scanning line before reception of the current picture element. This means that each of the picture elements must be precessed during each sample period in each of the predictive encoding device and the predictive decoding device to produce each local decoded signal or each local predictive signal. Accordingly, predictive encoding and predictive decoding operations of the input image signal should be carried out at a high speed in the predictive encoding device and the predictive decoding device. For this purpose, each of the predictive encoding device and the predictive decoding device must be implemented by a high speed processing device which is usually expensive and which has a large power consumption, so as to process such an input image signal. In other words, it is difficult to use a general-purpose processing device which is inexpensive and which has a low processing speed and a low power consumption.